1. Field of the Invention
The present invention relates to microwave amplification tubes, such as an inductive output tube (IOT), and, more particularly, to an input circuit for an IOT or other emission-gated device providing improved instantaneous bandwidth.
2. Description of Related Art
It is well known in the art to utilize a linear beam device, such as a klystron or traveling wave tube amplifier, to generate or amplify a high-frequency RF signal. Such a device generally includes an electron-emitting cathode and an anode spaced therefrom. The anode includes a central aperture, and by applying a high voltage potential between the cathode and anode, electrons may be drawn from the cathode surface and directed into a high-power beam that passes through the anode aperture. One class of linear beam device, referred to as an inductive output tube (IOT), further includes a grid disposed in the inter-electrode region defined between the cathode and anode. The electron beam may thus be density modulated by applying a radio-frequency (RF) signal to the grid relative to the cathode. After the anode accelerates the density-modulated beam, the beam propagates across a gap provided downstream within the IOT, and RF fields are thereby induced into a cavity coupled to the gap. The RF fields may then be extracted from the output cavity in the form of a high-power, modulated RF signal.
More particularly, an IOT, as well as other emission-gated microwave amplifiers, use density modulation to establish an AC current Jb on the electron beam directly at the cathode surface. This current is subsequently converted to RF energy through the Jb·Ec interaction with the output circuit field, Ec. Density-modulated amplifiers are highly efficient, even when operated in the linear region. Direct modulation of the beam at the cathode also enables compact device size.
In most density-modulated devices, RF gating of the electron emission is accomplished via an input cavity structure with a high-electric-field region situated between the cathode surface and a control grid. Energy from the signal generator is coupled into the input circuit, modulating the electron beam at the grid-to-cathode (g-k) gap. The basic elements of the input circuit are a resonant cavity, a coupled transmission line and a DC block. The gain-bandwidth product is limited by the interaction impedance R/Q·Q, where R/Q is the shunt impedance across the g-k gap, primarily determined by the gap geometry, and Q is the quality factor. The Q, proportional to the ratio of stored energy to dissipated power, determines the bandwidth of interaction between the drive signal and the electron beam. The power is dissipated by cavity ohmic losses, beam loading and external loading. The total Q is thus the parallel combination of the ohmic quality factor Q0, the beam loading quality factor Qb and the external quality factor Qext. When heavily loaded by the generator impedance through the transmission line, the cavity is strongly coupled and has a correspondingly low Qext. This reduces the total Q, which increases the bandwidth.
The input resonant cavity can be modeled as a parallel RLC circuit. The beam is included as a shunt impedance and the connection to the drive line is represented by a transformer with a turns ratio of N. The Qext is related to the turns ratio by:N2Z0=R/Q·Qext,where Z0 is the characteristic impedance of the input transmission line. Driven at its resonant frequency ω0, the cavity presents a purely resistive load of magnitude R/Q·Q to the signal generator, where R/Q is the shunt impedance across the g-k gap. As the drive frequency is shifted away from ω0, the load becomes increasingly reactive, and the resistive component decreases. At a small offset Δω from the center frequency, the load impedance is given by:
      Z    load    =      R          1      +              2        ⁢        j        ⁢                                  ⁢        Q        ⁢                                  ⁢                  Δω          /                      ω            0                              
When the real component of the load impedance has dropped to half of its value at resonance, or R/2, the power delivered by the generator will be halved. This occurs when Δω/ω0=1/(2Q). Hence, the fractional bandwidth of a resonant cavity, defined as the distance between the two half-power points divided by ω0, is given by the reciprocal of the total quality factor (1/Q).
The coupling transformer connecting the signal generator to the resonant cavity is typically implemented using an inductive loop to transfer power from the signal generator to the cavity. The degree of coupling is proportional to the ratio of the magnetic flux enclosed by the inductive loop to the total flux in the cavity. A resonant cavity is formed around the electron gun in the IOT, with the g-k gap supporting the electric fields that modulate the electron beam. The electron beam passing through the grid is bunched at the frequency of the input signal. Electrons are accelerated towards a positively biased anode before their energy is extracted by the output circuit. For existing IOT applications, such as UHF television broadcast, loop coupling provides adequate bandwidth of a few percent. Practical limits on the loop size prevent substantially larger bandwidths from being achieved. Hence, if a wide-bandwidth IOT were possible, the compactness and linearity of this device would make it an attractive option for many other applications.
Accordingly, it is highly desirable to improve the instantaneous bandwidth of the input circuit of an IOT or other density-modulated device.